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MGMAT Geometry Question Bank #9 (DS)

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alanaschorner
 Post subject: MGMAT Geometry Question Bank #9 (DS)
 Post Posted: Tue Jul 13, 2010 5:10 pm 
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Circular Motion:
Geometry Question Bank #9
A car is being driven on a road. Assuming that the car's wheels turn without slipping, how many full 360° rotations does each tire on the car make in 10 minutes?

(1) The car is traveling at 50 miles per hour.

(2) Each tire has a radius of 20 inches.

The correct answer is C, but I answered E because there is no information about how to convert inches to miles. I understand that it can be solved 50mph(1/6)=40pi(inches)*N*(?miles/?inches) but without knowing the conversion factor, I don't see how this can be solved. I know that the GMAT only expects you to know 1hr=60min and maybe 1ft=12in. So, for DS problems do we assume that the conversion factor is known even if it is not explicitly stated?
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dinesh19aug
 Post subject: Re: MGMAT Geometry Question Bank #9 (DS)
 Post Posted: Wed Jul 14, 2010 4:53 pm 
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Posts: 37
alanaschorner wrote:
Circular Motion:
Geometry Question Bank #9
A car is being driven on a road. Assuming that the car's wheels turn without slipping, how many full 360° rotations does each tire on the car make in 10 minutes?

(1) The car is traveling at 50 miles per hour.

(2) Each tire has a radius of 20 inches.

The correct answer is C, but I answered E because there is no information about how to convert inches to miles. I understand that it can be solved 50mph(1/6)=40pi(inches)*N*(?miles/?inches) but without knowing the conversion factor, I don't see how this can be solved. I know that the GMAT only expects you to know 1hr=60min and maybe 1ft=12in. So, for DS problems do we assume that the conversion factor is known even if it is not explicitly stated?


Yes Conversions are standard. In most Gmat questions, IF it is problem solving you will have conversion for terms which are not common, say nano seconds vs second, but for standard terms like distance: m -> cm, m->km Gmat does not give you the cpnversion value and assume that student already knows it.

However in questions like DS, you actually DO NOT need to solve the question. You only need to find if the "varaibles" required to solve the questions are present.

Solution:
n = number of turns
d = distance
r = radius
t = time

d = n * 2(pie)r
n = d/ 2(pie)r ==> (s *t)/ 2(pie)r

We already have "t" = 10 minutes so the only values that are required to solve n are s and r

Stmt 1: Speed is given, No info about Radius . INSUFFICIENT
stmt 2: Radius is given no info about speed. INSUFFICIENT

BOTH: Speed and Radius are given, we can solve for n. SUFFICIENT.
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debmalya.dutta
 Post subject: Re: MGMAT Geometry Question Bank #9 (DS)
 Post Posted: Wed Jul 14, 2010 5:21 pm 
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Posts: 47
alanaschorner wrote:
Circular Motion:
Geometry Question Bank #9
A car is being driven on a road. Assuming that the car's wheels turn without slipping, how many full 360° rotations does each tire on the car make in 10 minutes?

(1) The car is traveling at 50 miles per hour.

(2) Each tire has a radius of 20 inches.

The correct answer is C, but I answered E because there is no information about how to convert inches to miles. I understand that it can be solved 50mph(1/6)=40pi(inches)*N*(?miles/?inches) but without knowing the conversion factor, I don't see how this can be solved. I know that the GMAT only expects you to know 1hr=60min and maybe 1ft=12in. So, for DS problems do we assume that the conversion factor is known even if it is not explicitly stated?


To determine the number of 360 rotations , we need 2 things
1.the total distance covered
2.distance covered as part of one rotation ( circumference of the circle)

Statement 1 gives us only the total distance
Statement 2 gives us only the circumference of the circle

So.both the statements are required
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tim
 Post subject: Re: MGMAT Geometry Question Bank #9 (DS)
 Post Posted: Sat Sep 04, 2010 2:50 pm 
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Posts: 2242
Location: Southwest Airlines, seat 21C
good explanation, dinesh..

_________________
Tim Sanders
Manhattan GMAT Instructor
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siddharth.prahlad
 Post subject: Re: MGMAT Geometry Question Bank #9 (DS)
 Post Posted: Mon May 14, 2012 5:56 am 
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Is this question right? the radius of the tire is given not the radius of the circle. the circumference gives us the distance that the tire will travel in one rotation, it does not say in which direction. it could be in a straight line.

There is nothing that links the circumference to the center of the circle. both the statements are insufficient individually and together.

in ten minutes the car could be going straight.

either the question is wrong or the answer is (e)
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siddharth.prahlad
 Post subject: Re: MGMAT Geometry Question Bank #9 (DS)
 Post Posted: Mon May 14, 2012 7:13 am 
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Posts: 2
tim wrote:
good explanation, dinesh..


Is this question right? the radius of the tire is given not the radius of the circle. the circumference gives us the distance that the tire will travel in one rotation, it does not say in which direction. it could be in a straight line.

There is nothing that links the circumference to the center of the circle. both the statements are insufficient individually and together.

in ten minutes the car could be going straight.

either the question is wrong or the answer is (e)
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tim
 Post subject: Re: MGMAT Geometry Question Bank #9 (DS)
 Post Posted: Thu May 24, 2012 5:33 am 
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ManhattanGMAT Staff


Posts: 2242
Location: Southwest Airlines, seat 21C
the tire is the circle! and the connection is that once you know the radius you know the circumference. and once you know the circumference you know how far the car will travel in one rotation of the tires (it is equal to the circumference)..

_________________
Tim Sanders
Manhattan GMAT Instructor
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